For an electronic material, conductive paste having a certain level of viscosity prepared by adding and dispersing a conductive material in a binder resin, such as silicon, is mainly used. The conductive paste is then hardened for antioxidation by using a thermosetting resin, such as acryl, epoxy, or ester, before use.
For use as a conductive filler to be added to the conductive paste to provide conductivity, carbonaceous compounds, such as carbon black or graphite; various metal powder, such as silver or nickel; or various non-conductive powder or short fiber are surface-treated with metal, such as silver. In particular, silver paste using silver powder has a surface resistance of 10−4Ω/□ (square). Due to such a high conductivity, the silver paste is widely used as an electronic material in various fields.
However, such a paste using silver powder is relatively expensive (700 dollars/kg) compared to other metallic materials. Accordingly, copper paste has been introduced as an alternative to the silver paste due to its relatively low price. Compared to silver paste, copper paste has price competitiveness (7 dollars/kg) while having conductivity, a level similar to that of silver paste. Since copper paste has relatively high conductivity when price is taken into consideration, the use thereof as an electronic material is rapidly increasing due to its economic edge.
However, copper paste is highly likely to be oxidized compared to silver or other metals. That is, when copper paste is used as an electronic material and a long period of time elapses thereafter, oxygen in air or oxygen included in a binder resin may combine with copper powder to oxidize the surface of copper particles, leading to a rapid decrease in conductivity and losing its function as the electronic material. To solve these problems, a corrosion-protective agent is used to form a corrosion-protective coating layer on a copper surface. A representative example of the corrosion-protective agent is an imidazole compound. When a surface treatment agent containing an imidazole compound, such as 2-alkylimidazole, 2-arylimidazole, 2-alkylbenzimidazole, 2-aralkylbenzimidazole, or 2-aralkylimidazole, is used, the oxidation of a copper surface is effectively prevented. However, when the heat treatment temperature is 150° C. or more, a serious thermal history occurs. As a result, the formation of an oxidation film on the surface is promoted, and conductivity may rapidly decrease (Korean Patent Application No. 10-2011-7002997).
To obtain good conductivity, copper paste should have copper powder homogeneously dispersed in a binder resin. However, in the case of a conventional copper paste composition, the dispersion state of copper powder in a resin during mixing is not good, and accordingly, when used as an electronic material, corrosion may occur from where copper powder is not appropriately distributed (Korean Patent Application No. 10-2006-0137527).
Meanwhile, to apply copper paste to an electronic material, a pattern formation process is necessary. Conventionally, the pattern formation process is performed by photolithography, which is a semiconductor process technique. However, the photolithography has many limitations in terms of large-size patterning, a process price, and a process time. Accordingly, screen printing, which is a simple, low-cost, and environmentally friendly method, is used as an alternative to the pattern formation process based on copper (Current Applied Physics, 2012, vol. 12, pp 473-478). The environmentally-friendly or economic screen printing is distinguishable from an optical patterning in view that a large-size patterning is performed by using a small amount of conducting ink for a short period of time. Most of all, since conductive ink is efficiently coated on a flexible substrate through a fine mesh having openings for screen printing, the screen-printing can be used for various flexible electronic devices.
Accordingly, there is a need to develop a method of preparing copper paste that is corrosion-protective for a long period of time, and has excellent electric conductivity, heat resistance characteristics, and excellent dispersability of copper powder. In particular, to improve adhesive properties of a corrosion-protective layer on a copper surface by performing a silane treatment on an imidazole compound, maintain flexibility of a copper paste electrode, and have uniform resistance properties, there is a need to develop a method of preparing a binder by adding a high-viscosity polymer-based compatibilizer or a binder without an additional thermosetting process. In addition, the prepared copper paste is used as conductive ink for screen printing to simply and rapidly dope an electrode material on a flexible film, and then, a copper paste electrode having a target shape and size is manufactured for use in various electric/electronic devices.